Synthetic jet impingement heat transfer enhancement – A review

Coronavirus disease 2019 has become a global pandemic infectious respiratory disease with high mortality and infectiousness This paper investigates respiratory droplet transmission, which is critical to understanding, modeling, and controlling epidemics In the present work, we implemented flow visualization, particle image velocimetry, and particle shadow tracking velocimetry to measure the velocity of the airflow and droplets involved in coughing and then constructed a physical model considering the evaporation effect to predict the motion of droplets under different weather conditions The experimental results indicate that the convection velocity of cough airflow presents the relationship t-0 7 with time;hence, the distance from the cougher increases by t0 3 in the range of our measurement domain Substituting these experimental results into the physical model reveals that small droplets (initial diameter D ≤ 100 μm) evaporate to droplet nuclei and that large droplets with D ≥ 500 μm and an initial velocity u0 ≥ 5 m/s travel more than 2 m Winter conditions of low temperature and high relative humidity can cause more droplets to settle to the ground, which may be a possible driver of a second pandemic wave in the autumn and winter seasons © 2020 Author(s)

The motion of respiratory droplets produced by coughing

Coronavirus disease 2019 (COVID-19) has become a global pandemic infectious respiratory disease with high mortality and infectiousness. This paper investigates respiratory droplet transmission, which is critical to understanding, modeling and controlling epidemics. In the present work, we implemented flow visualization, particle image velocimetry (PIV) and particle shadow tracking velocimetry (PSTV) to measure the velocity of the airflow and droplets involved in coughing and then constructed a physical model considering the evaporation effect to predict the motion of droplets under different weather conditions. The experimental results indicate that the convection velocity of cough airflow presents the relationship $t^{-0.7}$ with time; hence, the distance from the cougher increases by $t^{0.3}$ in the range of our measurement domain. Substituting these experimental results into the physical model reveals that the small droplets (initial diameter $D \leq$ 100 $\mu$m) evaporate to droplet nuclei and that the large droplets with $D \geq$ 500 $\mu$m and initial velocity $u_0 \geq$ 5 m/s travel more than 2 m. Winter conditions of low temperature and high relative humidity can cause more droplets to settle to the ground, which may be a possible driver of a second pandemic wave in the autumn and winter seasons.

Volumetric particle image velocimetry (PIV) is a laser technique measuring three-component velocity field in a volume. Common volumetric PIV techniques are defocusing PIV, multi-view PIV and holographic PIV. Tomographic PIV is an advanced approach of multi-view PIV, which significantly improves the spatial resolution of the measurement by using the multiplicative algebraic reconstruction technique (MART) to reconstruct the particle field. Due to the rapid progress being made in volumetric PIV measurements, this article focuses on introducing the principles of different volumetric PIV methods from aspects of particle reconstruction and velocity reconstruction. Advantages and limitations on applications are elaborated. Issues specific to volumetric PIV regarding illumination, digital imaging, depth of focus, ghost particle and spatial resolution are discussed.

Review on development of volumetric particle image velocimetry

The singular value decomposition (SVD) and proper orthogonal decomposition are widely used to decompose velocity field data into spatiotemporal modes For noisy experimental data, the lower SVD modes remain relatively clean, which suggests the possibility for data filtering by retaining only the lower modes Herein, we provide a method to (1) estimate the noise level in a given noisy dataset, (2) estimate the root mean square error (rmse) of the SVD modes, and (3) filter the noise using only the SVD modes that have low enough rmse We show through both analytic and PIV examples that this method yields nearly the most accurate possible SVD-based reconstruction of the clean data Moreover, we provide an analytic estimate of the accuracy of this reconstruction

/pdf/singular-value-decomposition-of-noisy-data-noise-filtering-17gl8kxexx.pdf

Singular value decomposition of noisy data: noise filtering

For the first time, an experiment has been conducted to investigate synthetic jet laminar vortex rings impinging onto porous walls with different geometries by time-resolved particle image velocimetry. The geometry of the porous wall is changed by varying the hole diameter on the wall (from 1.0 mm to 3.0 mm) when surface porosity is kept constant ( increases to 0.20, the form drag associated with the porous wall geometry becomes significant.

Laminar vortex rings impinging onto porous walls with a constant porosity

Time-resolved particle image velocimetry was employed to study the effect of Reynolds number (Re
 sj) on synthetic jet vortex rings impinging onto a solid wall. Four Reynolds numbers ranging from 166 to 664 were investigated for comparison while other parameters were kept constant. It is found that the Reynolds number has a significant impact on the spatial evolution of near-wall vortical structures of the impinging synthetic jet. Velocity triple decomposition reveals that periodic Reynolds shear stresses produced by both impinging and secondary vortex rings agree well with a four-quadrant-type distribution rule, and the random velocity fluctuations are strengthened as Re
 sj increases. For radial wall jet, radial velocity profiles exhibit a self-similar behavior for all Re
 sj, and this self-similar profile gradually deviates from the laminar solution as Re
 sj is increased. In particular, the self-similar profile for low Re
 sj (166) coincides with the laminar solution indicating that periodic velocity fluctuations produced by vortex rings have little effect on the velocity profile of the laminar wall jet. This also provides evidence that the impinging synthetic jet is more effective in mixing than the continuous jet for the laminar flow. For the high Re
 sj, the mean skin friction coefficient has a slower decay rate after reaching peak, and the radial momentum flux has a higher value at locations far away from the impingement region, both of these can be attributed to the enhanced random fluctuations.

Experimental investigation of influence of Reynolds number on synthetic jet vortex rings impinging onto a solid wall

On the interaction between turbulent vortex rings of a synthetic jet and porous walls

Synthetic jet vortex rings impinging onto a porous wall: Reynolds number effect

Experimental investigation on the impingement of synthetic jet vortex rings onto a porous wall

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